1. Field of the Invention
This invention relates to imaging systems, and more particularly to a focal-plane detector imaging system that is resistant to potentially damaging high intensity optical beams.
2. Description of the Related Art
Imaging systems with focal plane detectors are used in devices that are designed to protect the human eye from exposure to potentially damaging optical radiation, particularly laser radiation. One such device is described in U.S. Pat. No. 5,081,542, entitled "LIQUID CRYSTAL LIGHT VALVE GOGGLES FOR EYE PROTECTION", issued Jan. 14, 1992 to Uzi Efron et al., and assigned to Hughes Aircraft Company, the assignee of the present invention.
In this device, an imaging lens images a scene under observation onto a photoconductive liquid crystal light valve (LCLV). The LCLV reproduces the imaged scene in a spectral range to which the human eye is sensitive. The observer views the reproduced scene and is thereby shielded from potentially harmful optical radiation.
The image detectors that are used in these eye-protection devices, such as the LCLV used in the device described above, typically have relatively low optical damage thresholds. Since the image detector is positioned at the image plane of the imaging lens, an incoming optical beam will be focused onto the image detector. The resulting optical intensity can very easily damage the detector and render the imaging device inoperative. One way to reduce the optical intensity at the image detector is to increase the diameter of the focused optical beam (increase the focal spot diameter), which can be accomplished by using an imaging lens with a high f-number. However, as the f-number of the imaging lens is increased, the field-of-view of the device is reduced.